The control of androgen production by the Leydig cell is directly regulated by luteinizing hormone via specific receptors for LH/hCG. In vivo treatment with endogenous or exogenous gonadotropin causes initial LH receptor up-regulation followed by down-regulation. Large doses of hCG cause "early" (prior to pregnolone) and "late" (17Alpha-hydroxylase, 17-20 desmolase) lesions that are independent of receptor loss or protein kinase activation. Such negative control by LH of receptors and steroidogenic responses is not observed in immature or fetal Leydig cells in vivo or in vitro, where only up-regulation was demonstrated. The goal of this project is to understand the steps involved in the control of steroidogenesis. We have extended our findings summarized above and have demonstrated that: a) in Leydig cells attached to a solid substratum, but not in cells cultured in suspension, hCG can induce acute up-regulation of the homologous LH receptors. This finding indicates that cell interaction with the extracellular matrix plays a role in the early up-regulation of LH/hCG receptors and of cell responsiveness to gonadotropin stimulation; b) hCG-induced early activation of RNA polymerase is mediated through nuclear actions of estradiol. This event could be related with the inhibitory action of estradiol on cytochrome P-450 dependent microsomal enzymes: c) 4-APP has a selective inhibitory action on the pituitary secretion of serum glycoprotein hormones; d) Leydig cells possess an active steroidogenic pathway from precursors prior to cholesterol. We will proceed with studies on the temporal, functional and biochemical definition of hormone-induced steroidogenesis and consequent enzymatic lesions, to help clarify the control mechanisms that regulate the activity of steroidogenic enzymes in the Leydig cell.